1. Field
This patent specification relates to an optical recording medium, and more particularly to a phase-change recording medium including a recording material capable of causing the phase changes by irradiating light beam to the material, thereby recording, reproducing and rewriting information data with improved characteristics and signal durability in repeated operations, which is utilized in optical memory devices such as, in particular, rewritable compact disks.
2. Discussion of the Background
Optical recording media have recently come into wide use as a viable information data storage and archival device of large capacity.
Of the optical disk storage devices, a phase-change type recording medium is capable of implementing repeated read/write/erase operations by means of laser beam irradiation utilizing the phase transition between amorphous and crystalline states. For this type of the media in particular, overwrite operations can be carried out using a single light beam with a simpler optical readout system, which can be utilized in, for example, rewritable compact disks (or CD-RWs). Phase-change type devices are advantageous over magneto-optical type memories which have difficulties in overwriting.
The phase-change recording medium has been attracting much attention recently for its capabilities described above. It has been disclosed in U.S. Pat. No. 3,530,441 to provide chalcogenide alloys for forming recording materials such as Ge—Te, Ge—Te—Sn, Ge—Te—S, Ge—Se—S, Ge—Se—Sb, Ge—As—Se, In—Te, Se—Te and SeAs.
Also disclosed to improve stability and crystallization speed are Ge—Te alloy materials added with Au (Japanese Laid-Open Patent Application No. 61-219692), with Sn and Au (Japanese Laid-Open Patent Application No. 61-270190), or with Pd (Japanese Laid-Open Patent Application No. 62-19490). Further disclosed to improve write/readout repeated operation characteristics are Ge—Te—Se—Sb and Ge—Te—Sb alloys with specified compositions (Japanese Laid-Open Patent Applications Nos. 62-73438 and 63-228433). These alloy materials, however, have not proven satisfactory in achieving various desirable characteristics of the rewritable phase-change optical recording medium.
In particular, there are problems of great importance yet to be solved to thereby achieve desirable characteristics for attaining sufficient sensitivity during either writing or erasing operation, preventing the decrease in erasure ratio caused by leftover portions during overwriting steps, and improving the durability of the media properties of either written or non-written portions in the recording medium.
Another recording medium is proposed in Japanese Laid-Open Patent Application No. 63-251290, including a single recording layer with a crystallized state of substantially more than ternary composition. By “substantially more than ternary” is meant in the disclosure that the alloy system includes at least 90 atomic % of a ternary compound (e.g., In3SbTe2) in the recording layer. It is also stated in the disclosure that write/erasure characteristics are improved with the alloy composition. However, this composition still has shortcomings such as erasure ratio of relatively small magnitude and laser power yet to be reduced for write/erase operations.
In addition, still another recording medium is proposed in Japanese Laid-Open Patent Application No. 1-277338, including (Sba Te1-a)1−yMy with 0.4≦a≦0.7 and y≦0.2, where M includes at least one additive which is selected from the group consisting Ag, Al, As, Au, Bi, Cu, Ga, Ge, In, Pb, Pt, Se, Si, Sn and Zn.
This alloy system is essentially composed of Sb2Te3 and several medium characteristics have been improved such as, high speed write/erase cycle operations by including a rather excess amount of Sb, and high speed erasure by the added M element. In addition, it is also stated that the erasing ratio is obtained to be relatively large for light beams in the continuous (or DC) mode. However, no description is found in that disclosure with respect to the erasing ratio for overwrite operations and that recording sensitivity is not satisfactory. In this context, it may be noted that leftover portions have been found by the present inventors during erasing experiments on this alloy system.
In a similar manner, further recording media are proposed including respective recording layers, in which the one recording layer includes (In1−x Sbx)1−yMy with 0.55≦x≦0.80 and 0≦y≦0.20, as disclosed in Japanese Laid-Open Patent Application No. 60-177446, where M includes at least one which is selected from the group consisting Au, Ag, Cu, Pd, Pt, Al, Si, Ge, Ga, Sn, Te, Se and Bi; the other recording layer includes an alloy GeTe—Sb2Te3—Sb(excess), as disclosed in Japanese Laid-Open Patent Application No. 63-228433. However, this recording media composed of these alloy systems have not attained sufficient media characteristics such as the recording sensitivity and erasing ratio.
Further, there are disclosed optical recording media provided with respective recording layers including alloy systems such as, a Ge—Te—Sb alloy added with N, described in Japanese Laid-Open Patent Application No. 4-163839; a Te—Ge—Se alloy formed such that at least one of constituent elements thereof is incorporated as a nitride, described in Japanese Laid-Open Patent Application No. 4-52188; and a Te—Ge—Se alloy adsorbed with N, described in Japanese Laid-Open Patent Application No. 4-52189. The optical recording media composed of these alloy systems, however, have not acquired satisfactory characteristics for the recording media.
In spite of numerous alloy materials for forming recording layers of the optical recording media, as described hereinabove, there persist needs to solve several problems of great importance and to thereby accomplish desirable media characteristics such as sufficient sensitivity during either writing or erasing operations by preventing the decrease in erasure ratio caused by leftover portions during overwrite steps, also improving durability of the structure and property of either written or non-written portions in the recording medium.
As for the optical media, compact disks (CDs) have come into wide use as viable information storage media. Along with the rapid growth of the CDs, another type of compact disk, which are writable only once, or CD-R's, have been developed and recently placed into the market. However, since information data on the CD-R disk cannot be corrected because of its write-once feature mentioned above, the CD-R disk has a shortcoming, in that the disk has to be abandoned when even one non-correctable error is inputted during the writing steps. Another type of storage medium has therefore been awaited for, that is capable of obviating the above disadvantage of the CD-R disk.
As an example of the storage media, there may be cited a rewritable compact disk utilizing magneto-optical materials. The magneto-optical disk, however, has drawbacks such as difficulty in overwriting and being incompatible with CD-ROM and CD-R disks. Therefore, a phase-change type recording medium has been actively developed recently toward practical use as one which features disk characteristics favorable to, among others, compatibility with the above media.
The research and development results disclosed so far on the rewritable phase-change recording media and compact disks incorporating the recording media are exemplified by Furuya, et al., Proceedings of the 4th Symposium on phase change optical recording (1992) 70; Kanno, et al., Proceedings of the 4th Symposium on phase change optical recording (1992) 76; Kawanishi, et al., Proceedings of the 4th Symposium on phase change optical recording (1992) 82; T. Handa, et al., Japanese Journal of Applied Physics, Vol. 32 (1993); Yoneda, et al., Proceedings of the 5th Symposium on phase change optical recording (1993) 9; and Tominaga, et al., Proceedings of the 5th Symposium on phase change optical recording (1993) 5.
These rewritable phase-change recording media, however, have not satisfied overall characteristics, such as the compatibility with CD-ROMs and CD-Rs, write/erase capability, recording sensitivity, repeatability of rewriting and readout operations, and stability during storage. The above noted shortcomings in the media characteristics are considered primarily due to relatively low erasure ratios which are caused by the composition and/or structure of the recording materials previously employed for forming the phase-change recording media.
Accordingly, it is desirable to develop novel recording materials capable of attaining higher erasure ratios and being suitable for more sensitive write/erase operations, to thereby be able to implement phase-change compact disks having improved rewritable capabilities. In order to find improved material systems and thereby solve the above noted shortcomings, the present inventors have previously proposed several AgInSbTe recording materials. These materials are disclosed in Japanese Laid-Open Patent Applications Nos. 4-78031 and 4-123551, and also described by H. Iwasaki, et al., Japanese Journal of Applied Physics, Vol. 31 (1992) 461; Ide, et al., Proceedings of the 3rd Symposium on phase change optical recording (1991) 102; and H. Iwasaki, et al., Japanese Journal of Applied Physics, Vol. 32 (1993) 5241. In addition, as a book of standards for the rewritable disk, Part III of Orange Book (Version 2.0) was published in 1998.
While Part III of the Book defines recording (or writing) steps on CD-RW disks with 1×/2×/4× linear speeds, it takes rather a long time for recording with such low velocities. Therefore, rewritable CD disks with higher recording velocities are desirable. In this context, it may be noted that CD disk characteristics at the lower recording velocities have been disclosed by the present inventor in U.S. Pat. No. 5,974,025, for example.
In addition, since the CD-RW disks are more expensive and less ease in use than CD-R disks, it is also desirable to overcome these drawbacks, to thereby be manufactured at lower costs. Further, it is also desirable to observe the recent trend to be more benign to the environment by conserving resources and energies in producing and using the recording disks.
As described above, by utilizing the AgInSbTe recording materials disclosed by the present inventors, CD-RW disks with 1×/2×/4× linear recording speeds have been known to be feasible having excellent disk characteristics. However, further improvements are desired to implement CD-RW disks with higher recording speeds.